This invention relates to an improved composition for use in forming shaped friction elements such as clutch facings and the like. More particularly, the invention relates to friction elements incorporating a specific combination of fibers in the form of twisted and plied yarns which provides improved friction characteristics and burst strengths. Such friction elements possess properties comparable with known friction elements yet, consonant with the increased demand to protect the environment against exposure to airborne substances that are deleterious to health, eliminates asbestos as the conventional component of such friction elements.
The mineral asbestos has been long associated with the manufacture of articles whose use requires that they withstand heat. Cloth woven from the fibrous substance was known, even in the 12th century, to remain incombustible when thrown into fire. It is not surprising, therefore, that asbestos has been the major component of friction elements such as those used in the brake and clutch assemblies of automotive vehicles where severe operating temperatures and pressures must be withstood. For more than 50 years, asbestos dominated as the principal active friction ingredient in friction materials.
When the asbestos used in friction elements is in the form of a yarn, the yarn is commonly provided with a core of fine metal wire to provide the tensile strength needed in handling the yarn during fabrication of a friction element. Asbestos fibers are commonly reinforced with a stronger fibrous substance such as cotton in order to spin the asbestos into a yarn. The introduction of cotton or some other fiber into the asbestos was to improve its spinability or the tensile strength of the yarn, and thus the addition was to be tolerated rather than encouraged. The need for including a limited amount of other fibers such as cotton in asbestos yarn intended for use in friction elements has become so well accepted that in recent years, such yarns are commonly described only as "asbestos yarn" without specifically mentioning such other fibers.
Since the passage of the Occupational Safety and Health Act of 1970, the standards for occupational exposure to asbestos have become increasingly rigid to the point where zero exposure may be required. The imposition of such standards was brought about by evidence that exposure to asbestos may be carcinogenic to man. It is thus a worthy (and perhaps in the future a mandatory) objective to eliminate asbestos as a component of friction materials.
The use of glass fibers in friction products has been suggested. Some of the earlier of these suggestions involved the use of glass fibers to reinforce asbestos-containing friction elements, first as backing materials for conventionally produced friction elements and then as part of the friction facing itself. The stated purpose for the use of glass fibers was to increase the burst strength of the friction elements (Burst strength is an index of the centrifugal forces which can be withstood by a friction element without disintegrating. The test of burst strength is usually carried out at elevated temperatures).
More recently, it has been suggested that glass fibers alone or together with metal wire or chips may be used to form friction elements containing no asbestos, as shown for example in U.S. Pat. Nos. 3,756,910 and 3,967,037. The glass fibers are said not only to improve burst stength and wear-resistance, particularly at elevated temperatures, but also to serve as the active friction ingredient. Generally, such friction elements are fabricated by known techniques using a conventional, heat-curable organic binder to bond the glass fibers together in a mass.
It has been stated that glass, as the active friction ingredient of a friction element, is too "aggressive" in most commercial applications in comparison with conventional asbestos-containing friction facings. This aggressiveness is often manifested during simulated or actual operating conditions as noise, vibration and/or erratic friction effects when the friction element is engaged with a mating surface, none of which manifestations can be tolerated by the highly-competitive automotive industry. One method for overcoming this aggressiveness is disclosed in U.S. Pat. No. 4,418,115, describing the use of a combination of glass fibers and acrylic or modacrylic fibers.
A variety of other methods for improving or modifying the friction and/or wear characteristics of asbestos-free friction materials are also known and further methods for modifying the properties of asbestos-free friction compositions are constantly being sought to widen the commercial applications for such materials.